1. Field of the Invention
The present invention relates to a combustion process and an apparatus therefor that provides means of introducing a fuel and an oxidant in separate streams in the combustion chamber of a furnace, so that the fuel burns with the oxidant in a wide, luminous flame, and whereby the combustion of the fuel with the oxidant generates reduced quantities of nitrogen oxides (NO.sub.x).
2. Related Art
Industrial high temperature processes, such as glass or frit melting, ferrous and non ferrous materials smelting, use large amounts of energy to transform a variety of raw materials into a hot molten product, that is then cast, formed or otherwise disposed of in further stages of the industrial process. This operation is generally performed in large furnaces, that can produce as much as 500 tons per day of molten material. Combustion in the furnace of a fossil fuel, such as natural gas, atomized fuel oil, propane, or the like, with an oxidant that contains oxygen is a preferred method of supplying the energy. In some cases, the combustion is supplemented by electric heating. Most of the time, the fuel and the oxidant are introduced in the furnace through burners, in order to generate flames. The transfer of energy from the flames to the material to be melted results from the combination of convection at the surface of the material, and radiation to the surface or into the material if it is transparent to the radiation. Flames that are highly radiant (usually referred to as luminous flames), are usually preferred, because they provide better heat transfer and, thus, higher fuel.
For flame heating, it is also very important to have the energy from the flame evenly distributed above the surface of the material to be melted. Otherwise, hot and cold regions may co-exist in the furnace, which is not desirable. The quality of products manufactured with material melted in such a furnace is often poor. For example, in a bath of molten glass, there may be glass stones in cold regions, and accelerated volatilization of glass in hot regions. Also, broad flames are preferred because they yield a better bath coverage.
In many countries, particularly the United States, increasingly stringent regulations are being promulgated regarding emissions of NO.sub.x. It is, therefore, important to develop combustion techniques wherein NO.sub.x formation is limited. In very high temperature processes, NO.sub.x. formation is promoted by long residence times of oxygen and nitrogen molecules in hot regions of the flame and the furnace. The use of substantially pure oxygen (about 90% O.sub.2 or higher) instead of air as the oxidant has proven to be very successful in reducing the NO.sub.x emissions by as much as 90%, since all nitrogen is eliminated. However, substitution of air by substantially pure oxygen increases the flame temperature, and thus creates regions in the furnace where the reactivity of nitrogen with oxygen is high, and wherein the formation of NO.sub.x may proportionally increase, even though it is globally decreased when compared to combustion with air. Also, it is impossible in practice to eliminate all nitrogen from a furnace, because industrial furnaces are not tight to air leaks, the fuel usually contains some nitrogen, and oxygen from non-cryogenic sources, such as oxygen produced by a Vacuum Swing Adsorption plant (VSA) contains a small residual nitrogen concentration.
Conventional methods of combusting fuel and oxygen for heating furnaces utilize post mix oxy-fuel burners. Conventional oxy-fuel burners have a metallic body with inlets for a fuel and an oxidant with a high concentration of molecular oxygen, and means to transport the streams with separate coaxially oriented channels to multiple injectors located at the burner tip. These burners generate high temperature flames with the shape of a narrow pencil at the burner tip, which needs to be located far enough into the furnace, to avoid or reduce overheating of the furnace walls. As a consequence of the high temperatures encountered in melting furnaces, one important drawback of these burners is the need for cooling, usually a jacket where a circulating fluid such as water provides the cooling. Such a burner is described, for example, in British Patent 1,215,925. Severe corrosion problems for the cooling jacket can arise particularly when the furnace atmosphere contains condensable vapors.
The gas cooled oxy-fuel burner is an improvement of the water-cooled burner. The body of the burner is protected from the furnace radiation by a refractory brick often referred to as a burner block, that possesses a substantially cylindrical cavity that opens onto the furnace. The burner is usually mounted at the back of the cavity, and it usually contains concentric injectors of fuel and oxidant located in the cavity, recessed from the furnace inner wall. The brick and the burner are cooled by a peripheral annular flow of gas, usually the oxidant gas. Such burners are described e.g. in U.S. Pat. No. 5,346,390 and U.S. Pat. No. 5,267,850. With this type of burner, combustion starts in the burner block before reaching the furnace. Thus, the flame is confined in and directed by the cylindrical cavity as a narrow axisymmetric jet, and provides insufficient covering of the melt in the furnace. These flames have high peak temperatures and generate relatively large amounts of NO.sub.x, because there is a direct contact between the oxygen and the fuel without dilution by the combustion products.
Another drawback of these gas cooled burners is that the flame may overheat and damage the furnace refractory wall because it starts in the wall itself. Also recirculation zones under the flame itself tend to accelerate refractory wear when the furnace atmosphere chemically reacts with the refractory material of the furnace wall which may reduce the furnace lifetime.
British Patent 1,074,826 and U.S. Pat. No. 5,299,929 disclose burners containing alternated multiple oxygen and fuel injectors in parallel rows in order to obtain a flatter flame. Although this brings an improvement in terms of coverage of the melt, these burners still produce relatively large amounts of NO.sub.x. Another drawback of these burners is that they are mechanically complex to build in order to obtain a flat flame.
It is also known to inject fuel and oxidant by streams separate distant injectors into a combustion chamber to generate flames detached from the furnace wall, with the aim of reducing refractory wear. One such apparatus is described in U.S. Pat. No. 5,302,112 wherein fuel and oxidant jets are injected at a converging angle into a furnace, which yields good mixing of the oxidant and fuel gases at the converging point of the two jets, thus enhancing the combustion rate but shortening the flame. However, the flame of such a burner has a high peak temperature and large quantities of nitrogen oxides are created in the furnace. To decrease this high peak temperature and significantly reduce formation of NO.sub.x it has been suggested in U.S. Pat. No. 4,378,205 to inject the fuel and/or the oxidant jets at very high velocities and to use separate injections of fuel and oxidant gases wherein the fuel and/or the oxidant jets entrain combustion products contained in the furnace atmosphere, and are diluted before the actual combustion between the fuel and the oxidant. However, the flames generated by these burners are almost invisible, as disclosed therein, col. 9, lines 58-65. It is, thus, extremely difficult for a furnace operator to determine and/or control the location of the combustion zones, and whether or not the burner apparatus is actually turned on, which may be hazardous. For certain applications such as glass melting, it is also generally recognized that luminous flames are desirable, because heat transfer from such flames is more efficient than for invisible flames. Another drawback of this burner is that the entrainment of combustion products promotes strong recirculation streams of gases in the furnace, which in turn accelerates the wear of the refractory walls of the furnace.
Another technique used to improve the heat transfer from a flame to a load is disclosed in U.S. Pat. Nos. 4,909,733 and 4,927,357, where a rate enhancing gas, generally oxygen, is injected through a non axisymmetric lance between a flame and the furnace load. With this technique, the flame temperature is increased, which results in higher nitrogen oxide formation. Also, according to the above cited inventions, the rate enhancing gas needs to be injected at high velocity in order to displace the flame towards the load. As mentioned before, this promotes strong recirculation streams of gases in the furnace, which in turn accelerates the wear of the refractory walls of the furnace.
Also, the use of high velocity oxidant jets requires the use of a high pressure oxidant supply, which means that the oxidant gas needs to be either produced or delivered at high pressure (the fuel gas is usually at relatively high pressure) or that the oxidant gas, such as the low pressure oxygen gas usually supplied by a VSA unit, has to be recompressed before being injected into the furnace.
Melting furnaces such as glass furnaces represent a high capital investment. Thus it is desirable to extend the lifetime of a furnace as much as possible while maintaining productivity. One of the aging factors of a furnace is superstructure temperature: for example, it has been demonstrated that the rate of wear and corrosion of a glass furnace crown was accelerated when the furnace was operated at high temperature. This can oblige the glass maker to repair the furnace prematurely, or to reduce the furnace pull rate at the end of the furnace campaign in order to prevent a catastrophic failure. In the case of a furnace equipped with oxy-fuel burners that produce generally high temperature flames, it is very important that the flames are not deflected towards the crown which would result in local hot spots. Such situations are known to occur for unstable flames that are deflected by the complex flow pattern of the combustion products in a furnace. For example, low momentum burners where the fuel and oxidant are injected at a low velocity in a furnace, overcome the drawbacks related earlier of high velocity burners, but tend to produce unstable flames. A combustion method that would prevent flame lofting and reduce furnace crown operating temperature would be particularly valuable for the industrials.
Thus, a need exists for a burner which may operate at low pressure, particularly for the oxidant gas, while producing a wide, flat, stable, luminous flame with reduced NO.sub.x emissions, and which affords a manner of controlling flame length so as to adapt the flame to the furnace in which it is used.